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Rules of Thumb (ROT)

January 21, 2019


Some Simple Guides


This page provides some tips and insights I've come across. A search on Google for “Rule of Thumb” leads to the following definition:


“A broadly accurate guide or principle, based on experience or practice rather than theory.”





Best Wastewater Treatment Option


Here is a favorite, very handy, ROT, quoted from the excellent reference provided below:


"Aerobic cultures of microorganisms are particularly suitable for the removal of organic matter in the concentration range between 50 and 4,000 mg/L as biodegradable chemical oxygen demand (COD). At lower concentrations, carbon absorption is often more economical, although biochemical operations are being used for treatment of contaminated groundwaters that contain less than 50 mg/L of COD. Although they must often be followed by aerobic cultures to provide an effluent suitable for discharge, anaerobic cultures are frequently used for high strength wastewaters because they do not require oxygen, give you less excess biomass, and produce methane gas as a usable product. If the COD concentration to be removed is above 50,000 mg/L, however, then evaporation and incineration may be more economical. Anaerobic cultures are also used to treat wastewaters of moderate strength (down to ~1,000 mg/L as COD), and have been proposed for use with dilute wastewaters as well. It should be emphasized that the concentrations given are for soluble organic matter. Suspended or colloidal organic matter is often removed more easily from the main wastewater stream by physical or chemical means, and then treated in a concentrated form. However, mixtures of soluble, colloidal, and suspended organic matter are often treated by biochemical means." 


Source: Grady, C.P. Leslie Jr., Glen T Daigger, and Henry C. Lim. "Biological Wastewater Treatment." Second Edition. New York: Marcel Dekker, Inc., 1999. 



It should be emphasized that the concentrations given are for soluble organic matter.





Based on the quote above from Grady, Daigger, and Lim, here is my attempt to provide a simple tabulation of the information.






Most of us are concerned with those things (solutes) added to water (the most common solvent) that are either soluble or insoluble. But you will also see the phrase "sparingly soluble" used and I've recently come across the best interpretation of these terms, taken from an excellent little book (shown below) I highly recommend for anyone who spends time in a laboratory. I quote the following from Mitchell's Laboratory Solutions book.


Solubility refers to the maximum amount of solute that dissolves in a given amount of solvent at a given temperature. Solubility is usually expressed by the number of grams of solute per 100 milliliters of solvent. Solubility, however, is also expressed in terms such as "soluble," sparingly soluble (or slightly soluble)," and "insoluble." These qualitative and somewhat subjective terms. As a guideline to these terms, not a strict definition, consider that a substance is:


(a) "soluble", if more than 1.0 gram of the substance dissolves in 100 milliliters of solvent;


(b) "sparingly soluble", if 0.1 to 1.0 gram of the substance dissolves in 100 milliliters of solvent;


(c) "insoluble", if less than 0.1 gram of the substance dissolves in 100 milliliters of solvent.


A dilute solution contains only a small amount of solute, whereas a concentrated solution contains a larger amount of solute for a given amount of solvent. The terms dilute and concentrated are relative, and therefore their use is somewhat ambiguous.


For a small but important set of chemicals, however, the term concentrated does have a definite meaning. These chemicals are all common acids and bases and usually exist in aqueous solution or as nearly pure liquids. For these chemicals, the term concentrated refers only to the typical values shown in the following table.



Source: Mitchell, Sharon Grobe. "Laboratory Solutions for the science classroom ." Batavia, Illinois: Flinn Scientific, 1995.



In addition to the limited categorization of solubility provided above, here's a more detailed breakdown reproduced from Sigma-Aldrich's website.





Effect of Temperature on Microbial Growth


In process control, accurate temperature measurements are helpful in evaluating process performance because temperature is one of the most important factors affecting microbial growth. Generally stated, the rate of microbial growth doubles for every 10 degree C increase in temperature within the specific temperature range of the microbe.


Source: United States Environmental Protection Agency. "Process Control Manual for Aerobic Biological Wastewater Treatment Facilities." EPA-430/9-77-006. March 1977.


An important point to note in the statement above is within the specific temperature range of the microbe.


Activated sludge systems typically operate in the Mesophilic range (see table below), though some industrial systems push into the Thermophilic range. What this means is that beginning at a temperature of 68 degrees F (20 degrees C) for the Mesophilic range, an increase in temperature in the bioreactor to 86 degrees F (an increase from 20 to 30 degrees C) will result in a doubling of bacterial growth. But this does not mean that a reduction in temperature to 50 degrees F (decreasing from 20 to 10 degrees C) will cause a 50% reduction in bacterial growth because 50 degrees F (or 10 degrees C) is outside the range for which this oft-stated temperature rule-of-thumb applies.



It should also be recognized that as the temperature in the bioreactor increases above 95 degrees F (35 degrees C), the growth rate doubling effect per 10 degree C increase will not continue to hold due to the high-temperature stress bacteria are being subjected to, a statement I am making based on my experience. To be clear, the temperature classification table reproduced above states the "optimum" temperature range for Mesophilic bacteria to be 25 to 40 degrees C (77 to 104 degrees F), but I have seen repeatedly that as the temperature climbs above 35 degrees C (95 degrees F) in the bioreactor, plant operating conditions will begin to deteriorate as evidenced but an increase in small, dispersed solids being lost from the secondary clarifier. 


For a more in-depth analysis of temperature please go to my blog post entitled "Wastewater Temperature."


TDS and Conductivity


The relationship between TDS (total dissolved solids) and conductivity depends on the water chemistry. For example, 1,000 mg/L of NaCl will give a different conductivity than 1,000 mg/L of MgSO4. The very rough rule of thumb is: TDS, mg/L × 1.6 = Conductivity (µS/cm). The factor of 1.6 used in the equation has a typical range of 1.4 to 1.8, though wider variations are certainly possible.


When possible, the best correlation is developed from the analysis of a specific water or waste stream for both conductivity and TDS from which a specific correlation factor is produced. Then, if the water chemistry remains fairly constant, conductivity can serve as a good indication of TDS. If the water chemistry changes significantly, the rule of thumb will not work.


Carbon Adsorption


Activated carbons, both powdered and granular, are made from a wide variety of carbonaceous starting materials: coals (anthracite, bituminous, lignite), wood, peat, coconut shells, etc. They are manufactured in such a way that they have a tremendous network of pores inside, and the total surface area inside such carbons is typically 500 to 1,500 square meters per gram, a huge amount. It is this extensive surface on which adsorption of organics can occur. Adsorption amounts up to as high as 0.30 g organic/g carbon are not unusual. 


Source: Cooney, David O. "Adsorption Design for Wastewater Treatment." Boston: Lewis Publishers, 1999.


Heavy Metals Impact on Activated Sludge


The table shown below, which can be found in another location on this site, is from a 1977 EPA manual called "Process Control Manual for Aerobic Biological Wastewater Treatment Facilities." I have reproduced the information which shows the allowable concentrations of 13 metals in the influent to an activated sludge process. I know a data of 1977 might be considered to be too out-of-date but this is the only source for this type of information I've been able to come across.


Activated Sludge Table


Below is a handy table reproduced from Metcalf & Eddy's Wastewater Engineering Treatment and Reuse textbook. If you click on the image below the table will open as a full-size page in the form of a PDF file for easier viewing and printing.





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